In general, nano materials correspond to media that have diverse shapes and physical properties without being limited to specific material areas of metals, ceramic polymers, biomaterials, etc, and influence nanotechnology directly and indirectly. Nanopowders are expected to be widely applied to electronic, photoelectron and magnetism fields, biomedical, pharmaceutical and cosmetic fields, and industry fields of energy, catalyst and structure.
Meanwhile, glass powders among various materials have been frequently used until now in diverse industry fields such as electronics and energy fields, and compositions and sizes of the glass powders are very diverse according to their application fields. For instance, the glass powders have been utilized in various component materials including partitions and electrodes for PDP TVs that made a rapid growth along with LCD TVs while replacing conventional cathode-ray tube TVs, and the glass powders have also been used as sintering additives for electrodes of silicon solar cells. Although micro-sized glass powders have frequently been used until now, it is required to develop nano-sized glass powders in the future for the purpose of accomplishing miniaturization, high efficiency and high performance of electrical and electronic products. Low melting point glasses are glasses which have an operating temperature of 500 degrees C. or lower and have been used as functional component materials and sealants for electronic products. Typical low melting point glasses include lead (Pb) based glasses. However, since uses of the lead (Pb) based glasses are strictly regulated by RoHS certification due to environmental pollution problems, the development of Bi-based glass compositions as an alternative to the lead (Pb) based glasses has actively progressed.
Preparation processes of such glass powders are conventionally divided into a wet type process and a dry type process, and basically include determining composition ratios of raw materials, measuring and uniformly mixing the raw materials, heating the mixed raw materials at a melting temperature or higher to prepare a glass melt, and rapidly cooling the melt to prepare small cullets, so that a milling process is performed on the cullets. Then, it is possible to obtain powders having a size from several millimeters (mm) to several hundred micrometers (μm). Such a milling process is often performed by ball milling, wherein powders having a size from several millimeters (mm) to several hundred micrometers (μm) obtained from the milling process are milled again to obtain powders having a size of smaller than 10 μm.
Here, the process has a demerit in that the glass powders are exposed to the pollution due to alien substances or solvent components remaining on surfaces of frits although the wet type milling process has good milling efficiency. The process has problems in that the configuration of initial facilities requires a high investment cost and it is difficult to lower particle sizes of the powders to not greater than 1 μm although the dry type milling process is preferred in material industries since the process does not have the problems of the wet type milling process.
Meanwhile, techniques of preparing nano glass powders using plasma have recently been introduced. Such techniques of preparing nano glass powders using plasma are dry type techniques having great merits that spherical nano powders may be obtained in large quantities relatively simply and there are not any pollution problems. Upon further discussing the thermal plasma process, non-transferred arc and transferred arc using direct or alternating current according to a plasma source may be used, and argon or nitrogen gas as a plasma-generating gas is mainly used. Spherical nano powders may be obtained by injecting raw materials into plasma in a solid, liquid or gas form to instantaneously heat the raw materials by high temperature plasma, performing melting and evaporation processes on the heated raw materials, and cooling the resulting materials very rapidly at a radical temperature gradient while passing through a reaction tube. As a present known technique, a technique of preparing nano glass powders for BaO-based dielectrics that do not contain Bi2O3 or PbO using an RF thermal plasma process is introduced in Korean Laid-open Patent Publication No. 10-2009-0067830. Further, a process of preparing relatively high melting point borosilicate nano glass powders containing SiO2 as a main constituent by using an RF plasma process is introduced in Korean Laid-open Patent Publication No. 10-2009-0075100.
As described above, although nano powders may be obtained in large quantities continuously and relatively easily by using the thermal plasma process, it is difficult to determine process variables exactly due to such a many process variables. Since process conditions may vary depending on the types of compositions, it has become a challenge that it is important to perform a quantitative comparison analysis on compositions before and after the process and variations in contents of the compositions.
Moreover, since a preparing technique of Bi-based nano glass powders, which have recently been received the limelight as an alternative to a PbO-based nano glass powder composition that is a conventional representative low melting point glass powder composition has not been introduced yet, it has been required to develop the technique.